When considering understanding the contact state of a contact surface using a tactile sensor, there are vectors of three components representing size and direction of force acting at each point of the contact surface. This is represented as f(x,y) in the coordinate system of FIG. 10. However, f is a vector, and so actually has three components x, y and z at each point. When explicitly expressing each component, it is represented as f(x,y)=[fx(x,y), fy(x,y), fz(x,y)].
Since force distribution has three components at each contact point, in order to reconstitute force distribution of a contact surface using a tactile sensor it is necessary to acquire information for each contact point on the contact surface with at least three degrees of freedom. However, a lot of surface type tactile sensors that are presently being used only acquire information for at best one degree of freedom for each point of the contact surface, as in cases where, for example, conductive rubber is used. Therefore, even if it is possible to ascertain the condition of a force generally acting and its distribution, it is difficult to determine whether this force is perpendicular to the surface, for example, or acting horizontally. This is because only a third of the information is obtained that would allow such determination to be made.
On the other hand, as specific sensing means, optical type tactile sensors have recently been attracting attention. With this method, differing from the conventional method where mechanical elements (such as piezoelectric elements) are used for direct measurement of stress, a sensor body is implemented as a transparent elastic body and a marker buried inside the transparent elastic body for causing variation in optical behavior using any force. The condition of the contact surface is then estimated by photographing behavior of the marker using an imaging system such as a CCD element. The advantage of this is that with the advancement of imaging elements in recent years they have become much cheaper than mechanical alternatives, and it is possible to construct high density tactile sensors. With this method, however, in a lot of instances this information has upward or downward displacement or pressure and the previously described problem where information is lacking is not resolved. Even with a conventional optical tactile sensor, derivation of only one type of information (one dimensional information) is achieved. The present invention has been conceived to resolve the drawbacks of conventional tactile sensors, and has as an object to provide a tactile sensor that can acquire information for a plurality of degrees of freedom at each point on a surface by introducing multi-channel sensing using color, that is, a light spectrum, to an optical tactile sensor.